Fix A/V sync error accumulation regression in avformat producer.
Fix A/V sync error accumulation regression in avformat producer.

Regresion occurred in commit 6cfe84 on March 18 when adding support for
libavformat v55.

wideioltd/mlt,wideioltd/mlt,xzhavilla/mlt,xzhavilla/mlt,xzhavilla/mlt,j-b-m/mlt,anba8005/mlt,xzhavilla/mlt,mltframework/mlt,siddharudh/mlt,anba8005/mlt,siddharudh/mlt,anba8005/mlt,j-b-m/mlt,anba8005/mlt,j-b-m/mlt,j-b-m/mlt,xzhavilla/mlt,mltframework/mlt,j-b-m/mlt,siddharudh/mlt,j-b-m/mlt,siddharudh/mlt,mltframework/mlt,xzhavilla/mlt,wideioltd/mlt,wideioltd/mlt,wideioltd/mlt,j-b-m/mlt,mltframework/mlt,zzhhui/mlt,zzhhui/mlt,wideioltd/mlt,wideioltd/mlt,siddharudh/mlt,mltframework/mlt,wideioltd/mlt,mltframework/mlt,xzhavilla/mlt,siddharudh/mlt,j-b-m/mlt,j-b-m/mlt,mltframework/mlt,xzhavilla/mlt,zzhhui/mlt,siddharudh/mlt,zzhhui/mlt,anba8005/mlt,mltframework/mlt,zzhhui/mlt,j-b-m/mlt,siddharudh/mlt,anba8005/mlt,anba8005/mlt,mltframework/mlt,zzhhui/mlt,zzhhui/mlt,siddharudh/mlt,zzhhui/mlt,zzhhui/mlt,mltframework/mlt,anba8005/mlt,anba8005/mlt,xzhavilla/mlt,wideioltd/mlt